xonen writes: It is now official — with the release of Unity3D version 4, Linux is an official target platform.

"Unity 4 Pro delivers the efficient and flexible workflow, graphical polish and power, smooth performance, fine tuned animations and creative control you need to make, ship and sell professional games."

The little catch here, is that Linux is not yet supported as host platform — you will still need a Windows or OSX computer to develop your games. However, we can expect a great deal of existing and new games to be released as Linux version too soon, especially since both the indie ('free') and pro versions of Unity3D support building for Linux target.